1. Field of the Invention
The present invention relates to porous polytetrafluoroethylene products and a process for their manufacture. More particularly, it relates to porous PTFE products having high porosity and very fine pores, i.e., less than 1,500A, and generally ranging from about 100 to 1,000A in pore size. The process for the manufacture of these products comprises shaping PTFE powder by extrusion and/or rolling, a first stretching step, followed by free sintering, and then a second stretching step. The porous PTFE products obtained by this process have microscopic pores uniformly distributed over the product. They may be suitable for use as a gas separation membrane (for inert gas, isotope gas, etc.), as an electrolytic diaphragm, a fuel cell separator, a filter, and such other uses as will be obvious to one skilled in the art.
2. Description of the Prior Art
Polytetrafluoroethylene (hereinafter referred to as "PTFE") has excellent heat resistance, chemical resistance, insulation resistance, non-adhesiveness, self-lubrication, and has found wide use in industry and in daily life.
In the field of porous PTFE product, several processing methods are known. These methods are divided into the following groups: (a) weaving or knitting PTFE threads or filaments into sheets; (b) a process comprising shaping of a PTFE mixture with extractable fillers, and then dissolving or extracting the fillers from the shaped mass to form pores; and (c) a process consisting of the shaping of a blended lubricant-containing PTFE by extrusion and/or rolling, followed by the subsequent stretching of the resultant sheet while in an unsintered or sintered state, the stretching being performed either before or after lubricant removal.
However, the above conventional process (a) is complex and expensive, and gives a product having large pores and irregular pore size distribution. The above process (b) requires not only extractants, but also costly production equipment, and the product obtained may contain residual extractant thereby affecting the chemical properties of the product. The above method (c) does not generally provide a film having fine pores, defined here to be less than 2,000A (0.2.mu.) in diameter.